STOCKHOLM — Americans James Rothman and Randy Schekman and German-born researcher Thomas Suedhof won the 2013 Nobel Prize in medicine today for discoveries on how hormones, enzymes and other key substances move in cells. This traffic control system keeps activities inside cells from descending into chaos and has helped researchers gain a better understanding of a range of diseases including diabetes and disorders affecting the immune system, the committee said. Working in the 1970s, 1980s and 1990s, the three researchers made groundbreaking discoveries about how tiny bubbles called vesicles act as cargo carriers inside cells. Above all, their work helps explain “how this cargo is delivered to the right place at the right time” the committee said. “Imagine hundreds of thousands of people who are traveling around hundreds of miles of streets; how are they going to find the right way? Where will the bus stop and open its doors so that people can get out?” Nobel committee secretary Goran Hansson said. “There are similar problems in the cell.” The discoveries have helped doctors diagnose a severe form of epilepsy and immune deficiency diseases in children, Hansson said. In the future, scientists hope the research could lead to medicines against more common types of epilepsy, diabetes and other metabolism deficiencies, he added. Rothman, 62, is a professor at Yale University, while Schekman, 64, is at the University of California, Berkeley. Suedhof, 57, joined Stanford University in 2008.